


The First Law of Thermodynamics

by Wildefire86



Series: Moments [8]
Category: Sonic the Hedgehog - All Media Types
Genre: Aged-Up Character(s), All aboard the feels train chooo chooo, Bring tissues, Established Relationship, Fluff and Angst, Husbands, Implied/Referenced Character Death, Love and Loss, M/M, This could wreck you, Time Travel, Writing this destroyed me, immortal Shadow
Language: English
Status: Completed
Published: 2021-02-26
Updated: 2021-02-26
Packaged: 2021-03-17 18:14:58
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,891
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29721240
Author URL: https://archiveofourown.org/users/Wildefire86/pseuds/Wildefire86
Summary: The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy can neither be created nor destroyed, but can be converted or transferred from one form to another.Maybe Shadow isn't the only one that's immortal....
Relationships: Shadow the Hedgehog/Sonic the Hedgehog
Series: Moments [8]
Series URL: https://archiveofourown.org/series/2027479
Comments: 18
Kudos: 58





	The First Law of Thermodynamics

**Author's Note:**

> This may not impact you like it did me. But it kinda destroyed me, I just wanted to give fair warning that, at least for me, this gave me a lot of feels. This is the final work in the Moments Series. I could still add to the existing ones, but this is the end of it. It feels right to end it here.
> 
> As always, Dumbledore-The-Phoenix is the best, most amazing beta ever. Please read her Cursed Child oneshot if you haven't, she is truly talented. <3
> 
> The idea for this story hit me about a week before I wrote it. I just didn't have the emotional reserves to put it into words. So I carried it with me for a few days until, finally, I got it out. I know it's a lot, but, if your age allows, I would really encourage you to read How Long Will I Love you, at least Chapter 1 of Chaos and Control, and especially the first chapter of As Long As Stars Are Above You. It's important for this, it will mean more if you do. 
> 
> Please give "I will spend My Whole Life Loving You" by Imaginary Future, Kina Grannis a listen while you read this. It's the song that made this idea pop into my head and it's very, very sweet. 
> 
> Ok. *big deep breath*. Here it is.

# The First Law of Thermodynamics

_The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy can neither be created nor destroyed, but can be converted or transferred from one form to another._

Sonic threw another pair of gloves in his bag before zipping it up and taking one last look around the bedroom he shared with his husband. The sun was setting over the ocean, brilliant hues of orange and hot pink casting their glow over the quiet space through the wall-sized windows. The gentle light reflected slightly over the platinum band on Sonic’s left ring finger, and he smiled at it lovingly, thumb tracing over the warm metal.

Shadow was on a mission for G.U.N., a particularly long one, and had already been gone for over a week. It was hard for Sonic to be in the big house by himself, his longing for the agent too much to bear in the empty silence, so, as was typical on occasions like this, he would stay with his little brother in the house they once shared together. 

He’d only come back to water Shadow’s plants and grab more clothes, hoping without reason that he might stumble in on his crimson-striped love starting dinner on his beloved stove, or stoking a fire in their stone-framed fireplace. 

But the house was quiet. Vacant. And Sonic could already feel the loneliness building again. 

With a final sigh, he hefted the bag onto his back, arms sliding through the straps, and made for the door and down the steps. He snagged an apple whose days were numbered from their fruit bowl and tossed it up in the air as he made his way back outside. 

He paused for a moment and took a bite, watching the sun dip further and further beneath the horizon when the fur on the back of his neck prickled. His eyes flickered to the side without moving his body. He’d been in the hero business way too long to not notice when he was being watched.

Without giving any indication he’d sensed anything at all, he remained where he was and listened for any unusual movement or sound from the treeline just beyond their house. With a touch more patience and self-control than he’d had in his younger years, he waited, slowly munching on the apple for--

There. The barest of rustles. Calculating the distance and angle effortlessly in his mind, he whirled around and threw the apple hard in the direction of the disturbance. 

“FUCK!” came the immediate response. A familiar response. In a familiar voice.

“Shadow?” Sonic mumbled in confusion before darting to the treeline. “Why are you--”

Something was different. Off. 

The black hedgehog rubbed his head where the apple had hit before absolutely freezing when he saw Sonic stood over him, and the look on his face…. It was the most emotion Sonic had ever seen the hybrid show at any one time. It was a flurry of thought and feeling that moved by so quickly that the hero didn’t have a chance to decipher any individual flicker. 

“Sonic…” Shadow said slowly, almost like his name was somehow foreign and strange on his tongue. Like he hadn’t said it in a while. 

Sonic took a step back, his shoes crunching the dried leaves of fall scattered across the ground, eyes narrowing suspiciously. Shadow was up immediately, hands out, pleading. “Don’t! Don’t go, I’m sorry--”

“Who are you?”

Shadow’s head whipped back like he’d been slapped, pain and fear flashing across his features. “You don’t--”

Sonic crossed his arms and tapped his foot. “I know you’re Shadow, but you’re not _my_ Shadow. _My_ Shadow wouldn’t be lurking in the bushes when we haven’t seen each other in over a week.”

The ebony hedgehog lowered his eyes, and Sonic could see in the orange light breaking through the trees all the little changes he hadn’t noticed at first glance. There, just across Shadow’s shoulder joint, was a new scar--or at least new to Sonic. The wound was clearly old and long healed. Another over his lower ribs on his left side, but there… on the ring finger of Shadow’s left hand, was the matching band to their set. His eyes grew wide, arms falling to his sides. “You are Shadow….”

“I am,” Shadow whispered, eyes still downcast. 

Sonic took a moment. He knew his husband inside and out, and… something was terribly wrong. His love hurt, he could feel it, see it in the way he held himself, the slumping of his shoulders, the dullness in his fur, the way he’d looked at him when he’d first broken through the trees….

“How far in the future are you from?” Sonic asked, putting the pieces together in his mind. 

There was a long pause before the dark hedgehog answered. “One hundred and forty-nine years.”

Sonic inhaled sharply, his legs feeling unsteady as he slipped the bag off his shoulders and set it on the ground beside his feet. “Shadow… what are you doing here?”

The black hedgehog sat back on the ground, dark fur contrasting with fallen leaves of maroon and gold. He looked up at Sonic with tear-filled eyes. “I couldn’t… I couldn’t do it.” He choked back a sob. “I couldn’t take it anymore. I had to see you.”

Sonic slowly moved next to him and sat down, the painted horizon just visible between the trees. “How long have I been gone?”

Shadow’s shoulders shook, and Sonic reached out to hold him without thinking. His partner latched on to him, clinging tightly while sobs wracked his body, fingers digging in almost painfully, like Shadow was afraid he was mirage that might slip away. Maybe that had actually happened before. Pain could do strange things. 

“It’s okay, it’s okay, I’m right here,” Sonic soothed, whispering against Shadow’s head. It took a while, but eventually a hoarse whisper made its way to Sonic’s ear. “I lost you forty-eight years ago.”

Sonic didn’t know what to say. They both knew going into this that someday… Shadow would have to go on without him. While Sonic grew old, the ultimate lifeform would remain frozen in time, his body automatically healing any signs of aging. They would get one lifetime together, granted an extended one for Sonic, considering the chaos energy coursing through his body, but still. One precious, fragile lifetime. 

Sonic felt his own tears drip down his face, the pain Shadow felt tangible, his heart breaking. 

“You told me, before you left, to keep living. To find joy. To keep fighting and protecting. And… I tried. I swear to you, I tried.” Shadow’s voice tapered off. “But, when you left, all the joy of living went with you. All the light. All the color. Everything good in the world… gone between one breath and the next.”

“You have to try again, Shadow,” Sonic said, sniffling as he held his love. “There’s so much good out there. You can find happiness again. I know it.”

Shadow sat back, and Sonic let his arms come back to rest in his lap, mirroring their positions all those years ago on the night it all started….

“How long will you love me?” Sonic asked softly.

Crimson eyes finally looked straight into his without hesitation. “As long as stars are above you.”

Sonic looked up at the faint stars just barely visible in the twilight sky. “Did you think that went one way?”

Shadow raised an eyebrow in question, and Sonic looked back at him and offered a sad smile. “You know, Knuckles told me once that there’s chaos energy in all of us, some more than others like you and I, but everyone has it. And just like with any other energy, it’s never lost or destroyed… sometimes it just takes another form. Me, my energy….” he reached for his husband’s left hand with his, lacing their fingers so their rings pressed together. “It never left you, Shadow. It’s still there. I’m still there. I’m just in another form.”

Shadow’s breath hitched as another sob shook his body, his eyes focused on their joined hands, and Sonic continued. “You still have stars one hundred and forty-nine years in the future?”

Shadow laughed through his tears and nodded. “Yes.”

“As long as stars are above you. That’s what we promised. I’m still there, Shadow. I’m still loving you. You’re not alone. You never will be. And someday, that part of me that can’t be destroyed will take yet another form. And when it does, I want you to find me, okay?”

Shadow ran his other hand over Sonic’s and shook his head. “There will never be anyone that could ever replace you.”

“Not replace me, Shadow. When you find me again, the person I am now will still be with you. All those memories and adventures, and let’s be honest, some pretty spectacular fights too.” Sonic chuckled through his own tears. “They’re all still there. You just make room for new ones. And your heart grows.”

Sonic reached up with his free hand and brushed away the tears still falling from Shadow’s eyes. “Every time you feel alone, or sad, or lost, I want you to think back to when I first told you I loved you. Think back to when you proposed to me, think back to the promises we made to each other on our wedding night. I want you to think back on all of it. Remember the races, the adventures. Remember all the times I annoyed the absolute shit out of you, and all the times we held each other close. Remember every single laugh and tear and “I love you,” and then ask yourself one question.”

Sonic bit his lip to hold back his own tears and looked at Shadow fiercely. “Was it worth it?”

Shadow’s face flooded with emotion. He leaned forward and pressed his forehead against Sonic’s, closing his eyes tightly. “Unequivocally.”

“Sonic?” A voice called out, and Sonic’s head whipped around back to the house. 

“It’s you,” Sonic whispered. “You’re back early.”

Shadow pulled back, a faint smile gracing his face as he looked toward the younger version of himself stepping outside the front of the house. “I remember. I was so desperate to get back to you.”

“I stay with Tails while you’re gone.... It’s too hard to be in the house without you. I miss you too much.”

Shadow cupped Sonic’s face reverently. “I know. Or knew. It was mutual, you know. Is mutual. I’ll miss you forever.”

Sonic smiled leaning forward slightly. “You don’t have to miss someone who isn’t gone. Remember that.”

He pressed his lips to Shadow’s, one last kiss for the love of his life. One last gentle reminder of the love that would follow him through lifetimes and never leave his side. 

As footsteps grew nearer, Sonic felt the warm hand against his face disappear, and in a flash of light, Shadow was gone. 

“Sonic?”

The blue hedgehog looked up as Shadow’s eyes landed on him in the growing darkness. “What are you doing out here? Are you okay?”

Sonic said nothing, just launched himself at his husband and embraced him tightly, lips seeking his desperately. “I love you,” he whispered through kisses. 

“As long as stars are above you.”

**Author's Note:**

> Part of me, a big part, wanted to make Sonic immortal. Let them live happily ever after, forever and ever, but.... there's a beauty to an end, you know? There's a quote from The Good Place (an awesome TV show if you don't know) that just really resonates with me...
> 
> _"Picture a wave in the ocean. You can see it, measure it, its height, the way the sunlight refracts when it passes through, and it’s there, and you can see it, you know what it is. It’s a wave. And then it crashes on the shore, and it’s gone. But the water is still there. The wave was just a different way for the water to be for a little while."_
> 
> Follow me @nottheweirdest1 on Twitter (NSFW Account) or @nottheweirdest on Tumblr for fanfiction updates. This fic is really close to my heart. If you have a moment, I'd love to hear your thoughts. <3


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